In copending U.S. application Ser. No. 602,670 filed Aug. 7, 1975 now abandoned and entitled "Method and Apparatus for the Manufacture of Glass", a rapid process of melting and refining glass is described in which a vitrifiable material is melted and brought to an elevated temperature while maintaining the viscosity of the molten mass at less than 1000 poises. As soon as the melting has been achieved, an intense foaming of the molten mass is effected throughout its entire thickness while keeping the viscosity at a value less than 1000 poises. The rate of expansion of the mass is at least 1.5 (preferably between 2 and 3). After the foaming subsides, a perfectly refined glass is collected.
According to the process disclosed in said copending application, the foaming operation is performed in a channel in which the molten material progresses, without back currents, from a first location where the raw vitreous material is received from a premelting apparatus and a second location where the refined glass is recovered.
To ensure the intense and complete foaming required, a number of steps may be taken. For example, foaming agents can be incorporated into the raw materials. The foaming agents give rise, in the temperature range, corresponding to the desired viscosities, to the formation of gas bubbles inside the glass. The gases produced by the foaming agents are soluble in glass, and preferably their solubility in the molten glass increases as its temperature decreases. It is also recommended that a refining agent be present, at least in the final phase. After the elimination of most of the gases, the refining agents aid in the readsorption of the bubbles which remain on cooling. The foaming agents are selected such that they do not induce foaming of the vitreous material until that material has reached a desired temperature, which temperature is maintained in the refining channel. The following foaming agents are useful in the process disclosed in said copending application: arsenic compounds, such as arsenic trioxide; antimony compounds such as antimony trioxide; sulfur compounds, such as sodium sulfate; and halogen salts such as potassium chloride. Other agents useful in the process will be apparent to those skilled in the art.
Another method disclosed in said copending application for ensuring the thorough foaming of the molten mass involves subjecting the batch to rapid uniform heating during the foaming operation of about 20.degree. C. per minute or more.
In a discontinuous melting installation, the heating means are employed at a time when the vitreous batch contains a large number of solid or gaseous nuclei and a sufficient amount of foaming agents to ensure an expansion of at least 1.5, and preferably above 2 times the normal volume of the mass in the unfoamed molten state.
In a continuous melting installation similar heating means can be employed. The predefined time sequence corresponds to the rate of treatment of the vitreous mass.
To aid the foaming process, it is also recommended that the raw materials contain a large number of nuclei, such as unmelted particules or small gas bubbles, capable of inducing the foaming. These nuclei essentially act as nucleation sites. The nuclei should be distributed throughout the molten mass at a concentration of at least 10 nuclei per cc. Generally, it is desirable that the raw materials be agglomerated. The agglomeration makes it possible to preheat the materials before actual melting. The preheating is accomplished by a brief and intense heat transfer (less than 10 minutes) while simultaneously keeping the temperature of the materials below the foaming temperature. This permits the maintenance of a high number of nuclei consisting of unmelted particles and gas bubbles in the vitreous mass introduced into the total foaming stage.
To assure the presence of sufficient nuclei, outside nuclei, for example, cullet or colored cullet can be added to the raw materials. In relation to the usual glass refining processes, the process disclosed in said copending application, requiring the presence of gas producing agents and foaming nuclei, can employ unrefined vitreous materials. It has been discovered that 1 to 2 mm. grains originating from the limestone and dolomite in the material introduced in the refining tank, are totally digested at the end of the total foaming phase. The process is therefore not dependent on the use of a vitreous batch of high quality.
The channel in which the molten mass flows can be of very simple geometry. Preferably, it has a slight width in relation to its length, in a ratio of 1:5 at least. This construction limits undesirable back currents. Also, for this same purpose, it is possible to use baffles, barriers, bottlenecks or even cascades along the path traveled by the vitreous molten mass during treatment in the channel of the refining apparatus.